Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colorful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colors which depend upon the materials employed. Examples of polymer-based OLEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixellated display. A multicolored display may be constructed using groups of red, green and blue emitting sub-pixels. So-called active matrix (AM) displays have a memory element, typically a storage capacitor, and a transistor associated with each pixel (whereas passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image). Examples of polymer and small-molecule active matrix display drivers can be found in WO 99/42983 and EP 0,717,446A, respectively.
Active matrix displays can be classified as either current programmed or voltage programmed, according to whether light emission levels are set by supplying data to pixels (through column or data lines) either as a current signal or as a voltage signal, respectively.
Background prior art relating to voltage-programmed active matrix driver circuits can be found in “The impact of the transient response of organic light emitting diodes on the design of active matrix OLED display” (Dawson et al, IEEE International Electron Device Meeting, San Francisco, Calif., 875-875, 1998). Background prior art relating to current-programmed active matrix pixel driver circuits can be found in “Solution for Large-Area Full-Color OLED Television—Light Emitting Polymer and a-Si TFT Technologies” (Shirisaki et al, of Casio Computer Co Ltd and Kyushu University, Invited paper AMD3/OLED5-1, 11th International Display Workshops, 8-10 Dec. 2004, IDW '04 Conference Proceedings pp. 275-278, 2004).
FIGS. 1a and 1b, which are taken from the IDW '04 paper, show an example current programmed active matrix pixel circuit and a corresponding timing diagram. In operation, in a first stage the data line is briefly grounded to discharge Cs and the junction capacitance of the OLED (Vselect, Vreset high; Vsource low). Then a data sink Idata is applied so that a corresponding current flows through T3 and Cs stores the gate voltage required for this current (Vsource is low so that no current flows through the OLED, and Ti is on so T3 is diode connected). Finally the select line is de-asserted and Vsource is taken high so that the programmed current (as determined by the gate voltage stored on Cs) flows through the OLED (IOLED).
The brightness of an OLED is determined by the current flowing through the device, this determining the number of photons it generates. An active matrix pixel circuit therefore must provide a means of controlling such a current through an OLED device. The setting of this current can be through the means of either a current or voltage programming signal. Voltage programming has the advantage of simplicity and speed but requires a reproducible relationship between the set voltage and the delivered current, a relationship which often changes with time. A current programmed circuit will copy the current onto the OLED and therefore does not rely on any indirect relationship and is therefore less prone to changes with panel age, however, current-programming methods exhibit longer settling times (charging times) due to the relatively large parasitic capacitance of the data lines.
“Acceleration of Current Programming Speed for AMOLED using Active Negative-Capacitance Circuit” (C.-H. Shim and R. Hattori, 14th International Display Workshops, December 2007, IDW '07 Conference Proceedings pp. 1985-1986, 2007) proposes the concept of “negative capacitance” to eliminate the effects of panel parasitic capacitance by implementing an equivalent circuit which has the same value as a parasitic capacitance but the opposite sign. FIGS. 2a and 2b, which are taken from the Shim and Hattori paper, show a simplified equivalent circuit of pixel driving transistor, parasitic capacitance, and negative capacitor for AMOLED, as well as a conceptual diagram of an implemented active negative feedback circuit. It is also known to pre-charge a display column.
There is, however, a need for improved techniques.